Keeping Your Legendary 855 Cummins Humming: The Lowdown on Valve Adjustment
Alright, let's talk about the Cummins 855. If you're running one of these beasts, whether it's powering a classic Peterbilt, a sturdy farm tractor, or a massive piece of construction equipment, you know it's a true workhorse. These engines are legendary for their reliability and power, often outliving the machines they're in. But even legends need a little TLC to keep them roaring efficiently, and one of the most critical, yet often overlooked, maintenance tasks is a proper 855 Cummins valve adjustment.
Now, don't let the technical sound of "valve adjustment" scare you off. It's not rocket science, but it is precision work. Think of it less like open-heart surgery and more like tuning a fine instrument. Get it right, and your 855 will sing its happy song – strong, efficient, and ready for whatever you throw at it. Get it wrong, or neglect it entirely, and you could be looking at power loss, poor fuel economy, or even some serious engine woes down the road. So, let's dive in and demystify this essential procedure for your trusty 855.
Why Bother? The Lowdown on Valve Adjustment for Your 855
You might be wondering, "Why do I even need to mess with the valves?" Good question! Inside your Cummins 855, you've got intake and exhaust valves constantly opening and closing, thousands of times per minute, letting air in and spent gases out. They're literally the breathing mechanism of your engine. For this breathing to be efficient and timed perfectly, there needs to be a very specific gap – called valve lash or clearance – between the rocker arm and the valve stem.
Over time, due to normal wear and tear, temperature fluctuations, and just the sheer amount of work these engines do, that clearance can change.
- If the clearance gets too tight: The valve might not fully close, or it might even stay open a tiny bit when it's supposed to be shut. This is bad news. You're effectively losing compression, robbing your engine of power, and worst of all, those valves can actually burn because they're not getting enough time to transfer heat to the cylinder head. That's a costly repair, trust me.
- If the clearance gets too loose: You'll likely hear it first – a distinct ticking or tapping noise, often called "tappet rattle." While not immediately as catastrophic as too-tight valves, loose valves lead to excessive wear on your valvetrain components, inefficient engine operation, reduced power, and generally just a less happy engine. Plus, that noise can get pretty annoying!
The bottom line is, a properly adjusted valvetrain ensures your engine breathes correctly, operates efficiently, maintains its power, and ultimately, lasts longer. It's preventative maintenance at its finest, ensuring you get maximum life and performance out of that legendary 855.
When Does Your 855 Need a Tweak?
So, how often should you be cracking open that valve cover? The general recommendation for the Cummins 855 series usually falls somewhere in the 15,000 to 25,000-mile range, or around 250 to 500 operating hours. However, and this is super important, always check your specific engine's service manual. Different applications or engine versions might have slightly different intervals.
Beyond the manufacturer's schedule, your 855 might be sending you some signals that it's time for an adjustment:
- Increased Engine Noise: That classic "ticking" or "tapping" sound we talked about. If it gets louder or more pronounced, that's a big red flag for loose valves.
- Reduced Power or Sluggishness: If your engine just doesn't pull like it used to, and you've ruled out other issues, valve lash could be a culprit.
- Decreased Fuel Economy: When the valves aren't working in harmony, efficiency goes out the window. You might notice yourself filling up more often.
- Hard Starting: Incorrect valve timing can make it difficult for your engine to fire up smoothly.
- Excessive Smoke: While many things can cause smoke, improper valve action can contribute to incomplete combustion and, you guessed it, smoke.
It's also a really good idea to check and adjust the valves any time you've done major work on the engine that could affect the valvetrain, like after a cylinder head removal, a camshaft replacement, or even just if you've had the rocker arms off for some reason.
Getting Ready: The Pre-Game for Your Valve Adjustment
Before you even think about grabbing a wrench, let's talk about preparation. This isn't a race; it's a careful process.
Safety First, Always!
- Disconnect the Batteries: Seriously. You'll be working around moving parts and electrical connections. Just disconnect them.
- Engine Cold: This is non-negotiable. Valves expand when hot, so adjustments made on a warm engine will be inaccurate once it cools down. Let it sit overnight if possible.
- Ventilation: If you're inside, make sure you've got good airflow.
The Toolkit You'll Need
You don't need a super exotic setup, but some specific tools are essential:
- Feeler Gauges: You'll need a set with the specific intake and exhaust clearances for your 855 Cummins. Common specs are often around .014 inches for intake and .027 inches for exhaust, but again, verify with your specific engine manual! Don't just guess or use a random number you found online.
- Wrenches: A good set of sockets and open-end wrenches for the rocker arm locknuts and adjustment screws. A torque wrench is also crucial for tightening everything down correctly.
- Barring Tool: This tool makes it much easier and safer to rotate the engine crankshaft by hand. You absolutely don't want to be fighting the starter or trying to turn it with a wrench on the crank bolt (though that's a common, if sometimes risky, workaround if you don't have a barring tool).
- Dial Indicator (Optional, but Recommended): For finding Top Dead Center (TDC) with absolute precision, especially for Cylinder #1.
- Marker or Chalk: Handy for marking cylinders as you go.
- Shop Rags and Cleaner: Things can get a little messy.
Clear out your workspace. Make sure you have good lighting. Lay out your tools. Having everything organized means less frustration and a more accurate job.
The Nitty-Gritty: Adjusting Those Valves
Okay, here's where the magic happens. We're going to use a common and straightforward method, often called the "two-rotation" method or a similar sequential approach, which makes it less confusing than trying to figure out valve overlap for every cylinder.
- Remove the Valve Covers: Take off those valve covers carefully. You might need to clean up some oil. Inspect the rocker arms and springs for any obvious damage while you're in there.
- Find Top Dead Center (TDC) for Cylinder #1: This is your starting point. Use your barring tool to slowly rotate the engine in its normal direction of rotation. As you rotate, keep an eye on the timing marks on the crankshaft pulley or flywheel. You're looking for the mark that indicates TDC for the #1 cylinder.
- Crucial Step: Once you're at the #1 TDC mark, you need to determine if it's the compression stroke (when you adjust the valves) or the exhaust stroke/overlap (when the intake and exhaust valves are both slightly open). You can usually tell by wiggling the rocker arms for cylinder #1. If both are loose, you're on the compression stroke. If one is pushing down or both feel tight (overlapping), rotate the engine another full 360 degrees, and you should be on the compression stroke.
- Adjust the "Set A" Valves: With Cylinder #1 at TDC on its compression stroke, you can adjust a specific set of valves. The Cummins 855 typically allows you to adjust a certain number of intake and exhaust valves at this initial position. Your manual will specify exactly which valves to adjust here. For example, it might be Intake valves 1, 2, and 4, and Exhaust valves 1, 3, and 5. Mark these off as you go.
- How to Adjust:
- Take the correct feeler gauge for the valve (e.g., .014" intake).
- Loosen the locknut on the adjustment screw for that rocker arm.
- Slide the feeler gauge between the rocker arm and the valve stem.
- Turn the adjustment screw until you feel a slight drag on the feeler gauge – not too tight that you're bending the gauge, and not so loose that it just falls through. It should slide with a consistent, gentle resistance.
- While holding the adjustment screw steady, tighten the locknut.
- Re-check the clearance! This is vital. Sometimes tightening the locknut changes the adjustment slightly. If it's off, repeat the process.
- How to Adjust:
- Rotate 360 Degrees for "Set B": Once you've adjusted all the "Set A" valves, rotate the engine another full 360 degrees in the normal direction of rotation. This brings Cylinder #6 (or the last cylinder in the firing order) to TDC on its compression stroke.
- Adjust the Remaining "Set B" Valves: At this new position, you'll adjust the remaining valves. Again, your service manual will list these specifically. For instance, it might be Intake valves 3, 5, and 6, and Exhaust valves 2, 4, and 6. Follow the same adjustment procedure as above, meticulously checking each one.
- Double-Check Everything: Before you button it back up, go back over every single valve you adjusted. Just a quick run-through to ensure the feeler gauge still slides with the correct drag. Trust me, it's worth the extra few minutes.
Common Mistakes and Pro Tips
- Rushing It: This isn't a job you want to hurry through. Take your time, stay focused, and be methodical.
- Not a Cold Engine: I can't stress this enough. If the engine's warm, your clearances will be off when it cools. Patience!
- Wrong Feeler Gauge Sizes: Always, always confirm the specs in your engine's manual. A few thousandths of an inch can make a big difference.
- Forgetting to Re-check: The number one reason for a poor adjustment is not re-checking after tightening the locknut.
- Not Using a Barring Tool: Trying to bump the engine with the starter is dangerous and imprecise. Invest in or borrow a barring tool.
Pro Tip: As you adjust each valve, put a small mark on the locknut with chalk or a marker. This helps you keep track and ensures you don't miss any or adjust one twice. After you're done, clean up any spilled oil, replace the valve cover gaskets (it's good practice to use new ones), and torque the valve cover bolts to spec. When you start the engine, listen for any unusual noises. A slight settling might occur, but it should generally sound quieter and smoother.
Wrapping It Up
Performing an 855 Cummins valve adjustment isn't the most glamorous job, but it's incredibly rewarding. It's an essential part of keeping your engine running like the champion it is, ensuring optimal performance, fuel efficiency, and a longer life for your investment. While it's certainly a DIY task for the mechanically inclined, if you're ever unsure, don't hesitate to consult a professional. They've got the specialized tools and experience to get it just right.
Your 855 Cummins has earned its legendary status through hard work and reliability. Give it the care it deserves, and it'll keep serving you faithfully for many more years and miles to come. Happy wrenching!